In digital cameras, video cameras, scanners, image recorders, and other devices for processing images in electronic format, an optical image is converted to electronic format by means of an image sensor, such as a CCD (charge coupled device) sensor or a CMOS sensor. Such an image sensor consists of several photosensitive picture elements which are preferably arranged in the form of a matrix. The number of picture elements in the image sensors affects the quality (resolution) of the image. Each picture element is subjected to a charge or another electrical quantity which is proportional to the quantity of light to which it is exposed and which can be measured. After the exposure to light, e.g. a mechanical shutter is used to prevent the access of light in the image sensor. The shutter function can also be implemented electrically by reading the image sensor sufficiently quickly, or by using, as the shutter, e.g. a liquid crystal display or another screen whose translucency can be electrically controlled.
The conversion of the analog signal generated by the image sensor to digital format can be made by an analog-to-digital converter. The accuracy of the analog-to-digital conversion is typically 8 bits, whereby each picture element yields 256 levels of luminous intensity. Considering the properties of the human eye, this number is normally sufficient to achieve the required image quality. From the analog-to-digital converter, this conversion result is transferred in parallel format to further processing steps, such as for recording in an image memory or on a videotape.
For the recording of images in digital format, several recording formats have been developed. These normally comprise a header part and an image data part. The header part normally comprises information about the image recording format, compression which the image may have been subjected to, the resolution, etc. One known image recording standard is the EXIF (Exchangeable Image File Format), which is applied e.g. in digital cameras by several manufacturers. According to this standard, the header part of the image record is used for recording, for example, data relating to the image size, the resolution, the time of recording, and the standard version. In compressed images, the header is normally also used for storing, for example, the information needed for decompression, such as a dequantizing table and a decompression table.
In many cameras, it is possible to determine auxiliary data to be recorded in the image, such as the time of capturing the image (date and possibly also the time). Furthermore, in some cameras, the user can determine a free-form text to be recorded on the image. This auxiliary data can then be seen in connection with the rest of the image data when the images are viewed later on. This arrangement has, for example, the drawback that the auxiliary information cannot be made invisible in the image but it is always displayed. If the user wants an image of an object where the auxiliary information is not visible, the user must then capture one image without the auxiliary information and another image with the auxiliary information. This is cumbersome and unnecessarily consumes the image memory of the camera. Moreover, the images taken of the same object one after the other are not necessarily fully identical. For example, such an arrangement is not suitable for capturing an object which is moving or changing in another way. Furthermore, the user may also want images with different auxiliary information. For example, the date is displayed in one image, text defined by the user is displayed in another image, and all the auxiliary information is displayed in a third image. When arrangements of prior art are used, this requires that the user takes several images and separately selects the auxiliary information to be included in each image, which makes the recording slower, consumes the memory capacity of the camera, and causes extra trouble for the user of the camera. To include desired auxiliary information in an image according to an image format, it is also possible to use image processing software. Also in this case, the user must process several image files. If standard image formats are not used, it is possible to use application-specific file formats, wherein it may be sufficient to process a single file. In such a situation, however, one must use the exact file format of the application in question. Examples of such applications include Photoshop® (trademark of Systems Inc.) and Powerpoint® (trademark of Microsoft Inc.).